Hadiya Masieh now devotes her time to speaking out against extremism. Photograph: Sarfraz Manzoor for the Observer

On Tuesday, the eve of the fifth anniversary of the 7 July bombings when 52 people died at the hands of Islamist terrorists, 100 orthodox Jewish and devout Muslim women will come together to try to foster closer ties between the two communities.

The women will gather at a west London cinema for a special screening of Arranged, a film about inter-faith friendship, before talking and learning about their respective lives. That the event is taking place at all is surprising, but what is even more startling is that the woman who has organised it was once a senior figure in a radical Islamist organisation.

Hadiya Masieh, a 32-year-old mother of three, spent 10 years as a member of Hizb ut-Tahrir, the controversial group that seeks an Islamic caliphate and has been accused of encouraging antisemitism. But then the terrible events of 7 July 2005 transformed her outlook. "I feel like I was a pawn," she told the Observer. "Led down the garden path when I didn't know better." Today Masieh devotes her time to speaking out against Islamist extremism.

As the anniversary of Britain's worst terrorist atrocity approaches, the story of her radicalisation provides a grim reminder of how a young person can lose her bearings. The way she managed to turn around her life in the aftermath of the bombings, even becoming an adviser to the last government on Muslim women's issues, is one of the few positive outcomes from a horrifying day.

In the autumn of 1996, Masieh was just another teenager leaving her home and family to start university. The boarding school-educated daughter of middle-class Mauritian and Ugandan parents, she had been raised as a Hindu but even before leaving to study in London she had begun to question her faith.

In the months before she left home, Masieh had begun to read and learn about Islam, and the more she studied it the more certain she became that Islam was the path she wanted to follow. Within months of arriving at Brunel University, she had converted to Islam and begun searching around at university for fellow Muslims. "I went looking for people who were religious," she says, "and they were the most prominent ones."

"They" were the members of Hizb ut-Tahrir (HT), the radical Islamist organisation which rejects democracy and campaigns for a single Islamic caliphate. "They would be in the prayer rooms and they seemed very religious," Masieh says. "I was a clean slate and I was impressed by how well they spoke. What they said seemed to make sense to me."

They told Masieh that the entire western world was out to destroy Islam and that the west feared the rise of Islamist ideology. "Because I was so ignorant and naive, what they said seemed persuasive," she says, "and it appealed to my political side. So before long I was spending every night with members of HT, getting more and more brainwashed."

The radical Muslims with whom she was spending ever more time became her substitute family. "We would talk and it wasn't just religion that we were discussing – it was about building a perfect world and establishing an Islamic state. My studies were secondary to what I was doing. I felt I was doing God's work."

After leaving university, Masieh became a full member of Hizb ut-Tahrir and it was through the organisation that she met her husband. "I was really into the thick of it by then," she says. "I was spending so much time trying to recruit people, going on marches, giving out leaflets, organising bazaars in Muslim areas as a way of getting people to join the organisation."

On 11 September 2001, when the United States was attacked by Islamic terrorists, it was evidence for Masieh and her fellow radicals that there were others who shared their anger at America and the west: "9/11 was great publicity for our cause," she reveals. "We found that more people were listening to our cause and when George Bush made his speech about being either with us or against us that just reinforced what our organisation had been saying."

But for Masieh 2001 was not only the year that the twin towers fell – it was also the year that she had her first child. "Having a baby meant I had less time to be active in the organisation," she says. "And I also moved out of London, so I wasn't mixing with the same crowd."

Once she was out of the Hizb ut-Tahrir cocoon, Masieh began to slowly question some of the certainties they had been preaching. It was during this time of doubt that Masieh, then heavily pregnant with her third child, went to her local hospital to check on the progress of her pregnancy. The date of her appointment was 7 July 2005. "I was in the hospital room having nurses doing tests on me but watching the television screen," she recalls. "It was then that it suddenly hit me that this was not a film – this was happening in London, the city I called home."

Two weeks later, on the day of the foiled second wave of terrorist attacks on the London underground, Masieh gave birth. The synchronicity of attempted murder and of new life left her convinced that she had to abandon any vestiges of her radical thinking and embrace a more tolerant version of Islam.

"The 7/7 bombers and the people I knew at HT were two sides of the same coin," she says. "HT says it does not believe in violence, but the violence was never condemned; they just didn't think it would achieve anything." She told the organisation that she no longer believed what it preached and she left.

These days Masieh works for the Three Faiths Forum, an inter-faith organisation that seeks to build bridges between religions. Her campaigning has led her to speak at universities, where she meets young Muslims who could be tempted to embrace radicalism.

"Because I have been there, I can counter the extremist arguments at a drop of a hat," she says. "I know how they work and operate and I can dismantle their arguments. If I had been offered alternatives, I would never have followed the path that I did."